TIME OF IRRIGATION 175 



111. Fall irrigation. Fall irrigation means irrigation 

 after harvest, but before winter sets in. After harvest, 

 water still flows down the river channels and ordinarily 

 goes to waste. This late water may be used to saturate 

 the soil, and thus be held over until the following growing 

 season. In districts where the fall and winter percipita- 

 tion is insufficient to saturate the soil, fall irrigation is 

 especially desirable. 



An average soil of the arid regions, under field con- 

 ditions, is saturated when it- contains about 18 per cent of 

 water to a depth of 10 feet. This is equivalent, approxi- 

 mately, to a depth of 3 feet of water. Under wise systems 

 of cropping, about 12 per cent of water is left in the soir 

 to a depth of 10 feet at the time of harvest. At the follow- 

 ing seed time, the soil should again be in a saturated con- 

 dition. The difference between 12 per cent and 18 per 

 cent, or practically 1 foot of water, should, therefore, be 

 furnished by the natural precipitation, or by fall, winter 

 or spring irrigation. 



Over a large area of the inter-mountain country, the 

 precipitation comes chiefly in the fall and winter or early 

 spring, that is, between harvest and seed time. In this 

 district, fall and winter irrigation have little value, if the 

 land is so treated as to permit the storage in the soil of 

 the rain- and snow-water unless the total seasonal 

 precipitation is low, when irrigation during the fall sea- 

 son may be very helpful. In other sections, much of 

 the rainfall comes in late spring, summer or early fall, 

 that is, during the growing season. This water does not 

 remain stored in the soil during the winter for the use 

 of next season's crop, and under this condition, fall 

 irrigation is highly profitable. 



It is of great advantage to have the soil saturated 



